Redundant prp transmission system

ABSTRACT

A method of operating a transmission system (1) having a first network (2) and at least one second network (3) where data is exchanged between these at least two networks (2, 3) in that data of the first network (2) is fed to duplicating means (4), then is transmitted wirelessly to separating means (5) via at least two transmission paths (6, 7) by PRP, and is forwarded by the separating means (5) to the connected second network (3), characterized in that the data is transmitted via the first transmission path (6) as data packets and, if a data packet was not transmitted, this data packet is retransmitted via at least the second transmission path (7).

The invention relates to a method of operating a data-transmission system having a first network and at least one second network where data is exchanged between these at least two networks in that data of the first network is fed to duplicating means, then is transmitted wirelessly to separating means via at least two transmission paths by PRP, and is forwarded by the separating means to the connected second network, according to the features of the preamble of claim 1.

Such known transmission systems are used in safety-critical applications in process plants, or stationary or mobile work facilities—for example in work vehicles such as cranes or the like.

It is important that data is reliably transmitted from the first network to the at least one second network. This manner of safety-critical data transmission is particularly important when the data is transmitted via a wireless transmission path. To this end, there has already been an improvement that consists of using not only one transmission path, but at least two, and preferably exactly two, transmission paths for this safety application. A further improvement to this redundant data transmission has been made by performing the transmission wirelessly—i.e. via radio or light—by PRP (parallel redundancy protocol) that is a layer-2 redundancy method that is independent of higher layers and, above all, is suitable for real-time Ethernet mechanisms.

From the perspective of safety, such a transmission system already operates satisfactorily, because there is redundancy in the two transmission paths. For example, if a wireless transmission path is disturbed or fails, the—at least—second transmission path can be used to ensure the data transfer from the first to the second network.

However, an unacceptable disturbance in data transmission between the two networks as concerns safety-critical aspects cannot be ruled out in spite of this redundancy.

On the one hand, the previously described data transmission via two, or more than two, transmission paths is advantageous with respect to the redundancy of data transmission from the perspective of safety. However, this parallel data transmission via at least two transmission paths results in higher energy consumption, and also leads to higher thermal stress on the components of the transmission system. This leads disadvantageously to reduced longevity of the power supply, particularly batteries, and can also reduce the life expectancy of the components of the transmission system considerably if they are operated under higher temperature conditions.

When data is transmitted via transmission paths by radio, such as WLAN, devices that work according to the 802.11 standard allow compensation for a higher loss rate on the wireless transmission path by repeating the transmission of lost packets on the layer 2 level. It is necessary in this case that the receiver of the data packets recognizes the reception of a data packet, and reports this information back to the transmitter of the data packet. However, this requires the sender of a data packet to know whether a data packet has successfully arrived at the receiver after it has been sent via the transmission path. Redundancy requirements are not taken into account in this type of data transmission, since only one transmission path is present.

The problem addressed by the invention is therefore that of improving a method of operating a transmission system, in terms of energy consumption, while simultaneously maintaining the redundancy characteristics for safety-critical issues.

This problem is addressed by the features of claim 1.

According to the invention, the data is transmitted as data packets via the first data path and then, if a data packet was not transmitted, this data packet is retransmitted via at least the second transmission path. The advantage of this is that data packets are transmitted in succession via the first data path, and each time that a data packet has been successfully transmitted via the transmission path, this is acknowledged by the receiver (separating means) and reported back to the transmitter (duplicating means). For the transmitter, this means that it is no longer necessary to resend a data packet via the first data path. Only once a data packet has been sent, but has not arrived at the receiver for whatever reason (for example because of a faulty transmission line), this is reported to the transmitter by the receiver that then transmits this data packet again via at least the second transmission path—that is, not on the transmission path on which the first transmission should have taken place. The receiver can determine for example, after a certain time has elapsed, that it has not received the data packet sent on the transmission path, and can then notify the sender that a data packet has been dropped, so that the same can once again transmit it via the—at least—second transmission path. If this retransmission of the data packet via the second transmission path is confirmed by the receiver, the sender is informed that this retransmitted data packet has arrived successfully and no retransmission of this data packet need take place. However, if there is a retransmission and the receipt of this retransmitted data packet does not occur, this as well can be reported by the receiver back to the transmitter, such that it again transmits the data packet that has already been transmitted unsuccessfully twice, via the second transmission path. This process can be repeated until a data packet has been successfully transmitted from the transmitter to the receiver.

As a complement thereto, in one implementation of the invention if a data packet has not been transmitted, this data packet is retransmitted, not only via the second transmission path, but via at least two transmission paths, and preferably via exactly two transmission paths. This manner of transmission of the data packets is particularly significant, on the one hand, with respect to redundancy, and on the other hand in terms of energy savings, because a data packet can in any case be reliably transmitted from the transmitter to the receiver, yet at the same time the number of data packets that must be transmitted is reduced. At this point, reference is made to FIGS. 2 and 3 that illustrate the difference in the number of data packets transmitted, comparing the prior art to data transmission according to the invention.

In one implementation of the invention, the retransmission of a data packet occurs more than twice. This ensures that a data packet is transmitted until the data packet transmitted by the transmitter arrives successfully at the receiver. In terms of energy conservation, the number of retransmissions can be limited. This means that, in one implementation of the invention, the retransmission of a data packet is suppressed if the transmission of this data packet is carried out without error. As regards energy consumption, the number of retransmissions of a data packet is intended to be 2, 3 or 4. Three attempts at transmission of a data packet is particularly advantageous, as this represents an advantageous compromise in terms of energy consumption, on the one hand, and on the other hand reliability and/or redundancy—as well as the performance of data transmission.

The presented method can be applied to a transmission system as shown in FIG. 1.

FIG. 1 shows a basic arrangement of a transmission system comprising two networks 2 and 3 that are intended to exchange data. This data exchange can either be unidirectional from the network 2 to the network 3 (or vice versa), or can be bi-directional between the two networks 2 and 3.

The networks 2 and 3 can be simple or complex networks, for example in a ring or linear topology or the like. However, it can also be contemplated that such a network 2 or 3 comprises only a single element, such as a sensor, an actuator, a controller or the like.

To transmit the data of the network 2, for example to the network 3, duplicator 4 is provided. This duplicator 4 divides the supplied data stream into two data substreams. Then, the two data substreams are merged after their receipt by a separator 5, and the received data streams are forwarded to the network after merging. The transmission of data between the duplicator and the separator 5 occurs wirelessly, by PRP, via two identical or different transmission paths 6, 7. It can also be contemplated that one transmission path 6 is a radio transmission path, and the second transmission path 7 is an optical data transmission path. If both transmission paths 6, 7 are radio transmission paths, for example, the data—and more specifically, the data packets—can be transmitted via these two radio transmission paths, for example on the same frequency or different frequencies, and with otherwise identical or differing transmission parameters. Identical transmission paths a 6, 7 are preferable in terms of their structure, although different transmission paths 6, 7 (for example optical/radio, or different transmission parameters) are preferable in terms of increasing redundancy.

After the data has been relayed by the first network 2 to the duplicator 4 (where PRP is used, also termed the redundancy box), it functions such that each data packet is transmitted several times over the same transmission path 6, 7 and/or an error correction value is assigned to each data packet. Subsequently, in a corresponding manner, the data packets are transmitted via the transmission paths 6, 7, and are accordingly evaluated by the separator 5 (where PRP is used, also termed the redundancy box), optionally processed, and relayed as data packets to the second network 3.

The above description of FIG. 1 relates to a unidirectional data transmission from the first network 2 to the additional, in particular the second, network 3. For this purpose, the duplicator 4 are designed to divide the data stream, and the separator 5 to merge the received data stream.

If data transmission from the network 3 to the network 2 is desired, further a duplicator 4 and/or separator 5 can be included in the transmission path between the network 3 and the network 2, such that there is a doubled structure. Alternatively, the means 4, 5 can also be designed to double not only the relayed data stream, but also to separate the data streams relayed via the transmission paths 6, 7, which also applies to the separator 5.

FIG. 2 shows the manner in which data packets are transmitted by devices that work via WLAN—that is, according to the 802.11 standard. These devices that use the 802.11 standard, are suitable for and designed to retransmit data packets on the second layer to compensate for loss of data packets.

The manner of the retransmission of lost data packets with respect to the second layer is shown in FIG. 2. The figure shows that, if a data packet was not transmitted, this data packet is retransmitted via the at least two transmission paths (the upper and lower transmission paths in FIG. 2).

In this case, it is assumed that the first data packet “1” was sent via the upper transmission path, but has been lost. For this reason, it is marked with an X. This causes the transmitter to retransmit this data packet “1” via the upper transmission path. At the same time, it is also retransmitted via the further transmission path (lower transmission path). Since the upper packet data “1” arrives first at the receiver that is not shown, the data packet “1” on the lower transmission path can be rejected by the receiver. The data packet “2” is transmitted on the upper transmission path and arrives successfully at the receiver. This is reported back by the receiver to the transmitter, and a repeated transmission of the data packet “2” on the upper transmission route can be suppressed. However, since a certain time is needed until the data packet “2” arrives at the receiver and this receipt is acknowledged by the same, the transmitter has already had the occasion to transmit the data packet “2” on the lower path. Since it has successfully arrived in the meantime via the upper transmission path, the retransmitted data packet “2” was able to be rejected on the lower transmission route. After data packet “2”, a data packet “3,” was transmitted on the upper transmission path, and was lost. It was then retransmitted on the upper transmission path. As it was lost again (that is, no successful data transmission occurred for a total of three times), the transmission of the data packet “3” on the lower transmission path was initiated. It then successfully reached the receiver, and this was acknowledged by the receiver such that a retransmission of the data packet “3” was suppressed. After that, the data packet “4” was transmitted on the upper transmission path. Since a certain amount of time was needed in this case as well before it was received by the receiver and the successful reception was acknowledged, it was retransmitted via the lower transmission path again. This retransmission can, however, be discarded.

This above-described manner in which the data is transmitted can be repeated for each subsequent data packet, according to whether a data packet has been successfully transmitted or not.

Because a certain amount of time is always required to transmit a data packet sent by the transmitter via the transmission path to the receiver, and a certain amount of time is also required by the receiver to report the successful reception back to the sender, it can also be contemplated that a data packet is first transmitted via the second transmission path. This is the case in the example according to FIG. 2, with data packet “5”. Because in this case the data packet “5” transmitted first has been lost, it is retransmitted on this transmission path. Alternatively, it can be contemplated that it is resent on the upper transmission path after the first delivery on the lower transmission path. In the case shown in FIG. 2, the data packet “5” resent on the lower transmission path arrives successfully at the receiver, such that retransmission does not occur. Also, for another data packet “6”, that was sent on the upper transmission path, that initially transmitted data packet “6” is lost, such that the same procedure is carried out as has been described above for the packet data “1”. Due to the different possibilities shown in FIG. 2, data packets that are lost on one and/or other transmission paths can be successfully transmitted between the transmitter and the receiver (that is, the duplicator 4 and the separator 5 according to FIG. 1).

From the perspective of reducing energy consumption, the type of data transmission according to FIG. 3 is of particular interest and particularly advantageous.

In this method, the data is once again initially transmitted as data packets via the first (upper) data path. Since the first data packet “1” was not successful by the transmitter and receiver, this data packet is again transmitted via at least the second (lower) transmission path, but optionally also via the upper transmission path. If the receiver recognizes that the data packet “1” was successfully transmitted either via the upper transmission path or, preferably, via the lower transmission path, this is reported back to the transmitter. The same sends a further data packet “2” on the upper transmission path. This data packet “2” is successfully transmitted, such that the transmission of the next data packet “3” can then be initiated. Because this data packet “2” is lost on the first transmission path, it can be retransmitted on the upper transmission path and/or the lower transmission paths thereof. In the example according to FIG. 3, the data packet “3” is lost on the upper transmission path, but is retransmitted successfully one the lower transmission path. Once this has been determined by the receiver and reported back to the transmitter, it allows the transmission of the next data packet “4”. This is successfully transmitted via the first data path, such that a retransmission can be suppressed, on whichever of the transmission paths. The same applies for the following data packet “5”. The same procedure is used for the following data packet “6” as that illustrated and described in FIG. 3 with respect to the data packet “1”.

As can be seen by examining FIG. 3, the number of transmitted data packets, in particular on the lower transmission path, is significantly reduced such that a significantly reduced energy consumption and a lowering of the operating temperature is achieved as a result. At the same time, however, it can also be seen that all data packets “1” to “6” were reliably transmitted from the transmitter (the first network 2) to the receiver (the second network 3). This approach therefore achieves a reduction in energy consumption, reduction in the operating temperature, and a redundant data transmission, in a particularly advantageous manner as concerns safety-critical aspects.

List of reference numbers 1 transmission system 2 first network 3 second network 4 duplicating means 5 separating means 6 first transmission path 7 second transmission pat 

1. A method of operating a transmission system having a first network and at least one second network where data is exchanged between these at least two networks in that data of the first network is fed to duplicating means, then transmitted wirelessly to separating means via at least two transmission paths by PRP, and is forwarded by the separating means to the connected second network, the method comprising the steps of: transmitting the data via the first transmission path as data packets and, if a data packet was not received, retransmitting the unreceived data packet via at least the second transmission path.
 2. The method according to claim 1, wherein, if a data packet was is retransmitted, it is retransmitted via the second transmission path.
 3. The method according to claim 1, or 2, wherein the retransmission of a data packet is carried out more than two times.
 4. The method according to claim 1 further comprising the step of: suppressing retransmission of a data packet if the transmission of this data packet is carried out without error. 